The invention relates to a method of making semiconductor device structures using at least one ion implantation process for introducing dopants into a silicon semiconductor body.
Ion implantation has become an essential process in the production of semiconductor devices and integrated circuits where it is used to advantage for doping semiconductor regions. Besides various other applications it is also used, for instance, to trim integrated resistors, to adjust threshold voltages of FETs, etc.
Prior to performing an ion implantation process it is known to initially apply a protective layer to the surface area of the semiconductor body in which implantation is to be effected. For this purpose, a silicon dioxide layer with a thickness of about 10 nanometer (nm) is generally used. On the one hand, this protective layer serves to randomly scatter the ions impinging upon the surface of the semiconductor body, thus preventing these ions from deeply penetrating the material along preferred crystallographic directions (channeling). On the other hand, the whole semiconductor substrate is covered by the protective layer during the penetration of the dopants, so that no part of the substrate is subjected to the contaminating effects of the ambient atmosphere at any time. The ions of a contaminating material, which largely result from collisions between the doping ions of the ion beam and the ion implantation device, i.e., the lateral walls or slits shaping and forming the beam, are screened by the protective layer. Contaminations are, for example, atoms or molecules of iron, nickel, chromium, manganese or aluminum, as well as oil from the vacuum pumps. In many instances the contaminations originate from the materials used for the mask, such as carbon given off by the photoresist materials.
Silicon dioxide is suitable as a protective layer because of its compatibility with the semiconductor processes generally used. So far, protective layers have been produced from silicon dioxide by subjecting the areas of the silicon wafer to be covered with the protective layer to a dry thermal oxidation step. To this end, the semiconductor wafers were placed in an oven, where they were heated for about 15 to 30 minutes in an oxidizing atmosphere to temperatures of the order of 900.degree. C. During this treatment a silicon dioxide layer with a thickness corresponding to the time of treatment was formed on the silicon wafer.
It is also known (U.S. Pat. No. 3,563,809) to produce implantation protective layers by direct bombardment of silicon with oxygen ions in an ultra-high vacuum. However, this method has the disadvantage that the silicon dioxide is formed only after a relatively long implantation time of about 20 to 40 minutes. This is due to the fact that all the oxygen is initially implanted into the bulk of the silicon crystal, so that the oxygen concentration at the silicon surface is small, reaching a constant maximum value only as a result of sputtering off after some time. This leads to a retarded oxygen enrichment at the crystal surface.
It is the object of the invention to provide a method for producing semiconductor devices, whereby an implantation protective layer is formed more rapidly than has been possible in accordance with the art and whereby an additional process step is saved.